Nuclear waste immobilization into structure of zirconolite by Complex Sol Gel Process
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Nuclear waste immobilization into structure of zirconolite by Complex Sol Gel Process Tomasz Smoliński1, Andrzej Deptuła1, W. Lada1, Tadeusz Olczak1, Andrzej G. Chmielewski1, and Fabio Zaza2 1
Institute of Nuclear Chemistry and Technology (INCT), 03-195 Warsaw, Poland ENEA-Casaccia Research Centre, UTPRA-GEOC SP011,Via Anguillarese 301,00123 Rome Italy 2
ABSTRACT Zirconolite (CaZrTi2O7) is one of the components of Synroc materials, which are regarded throughout the world nuclear as the second generation of high-level nuclear waste forms. The zirconolite phase was synthesized by a sol-gel method, with one variant of the method making use of ascorbic acid as a strong complexing agent. Into the structure of the zirconolite was incorporated 10 mol% Sr. Undoped and doped samples were examined by thermal analyses and X-ray diffraction. Addition of ascorbic acid to the sols lowered the firing temperature and promoted formation of the zirconolite phase. INTRODUCTION Synroc, a kind of synthetic rock, was invented by Ringwood in 1978 [1,2]. It is regarded as the second generation of high-level waste (HLW) immobilization forms. Because of high resistance to leaching and good durability, it now appears to be the best solution for immobilizing HLW [3,4]. Synroc is a ceramic composed of a variety of geochemically stable, naturally occurring minerals [1-5]. Nearly all of the elements present in high-level radioactive waste can be incorporated into these crystalline minerals [4]. Synroc can take many forms, depending on the type and form of waste that is incorporated. For example, Synroc C consists of the following ceramics: perovskite (CaTiO3), zirconolite (CaZrTi2O7), and hollandite (BaAl2Ti6O16). Long-lived actinides such as Pu can be immobilized in perovskite and zirconolite, which can also immobilize many fission products. In hollandite, elements such as Cs, K, Rb, and Ba can be immobilized. The most common method of production of Synroc is synthesis by solid-state reaction. Radioactive waste elements are added into the obtained matrices, and the resulting materials are then pressed and sintered. An alternative solution for synthesis of the various Synroc materials is the sol-gel method. This method allows for direct incorporation of radioactive elements into the mineral structure during its formation from an aqueous phase [6]. The inherently homogeneous distribution of the components in a gel should, in comparison with solid-state methods, reduce sintering temperatures and increase the resistances of final products to leaching of radioactive elements. In this work, our patented complex sol-gel process [7,8] was used to synthesize zirconolite phase capable of incorporating significant concentrations of high-level waste. This process makes use of ascorbic acid (ASC) as a strong complexing agent. The goals of this work were to synthesize zirconolite, to incorporate Sr as a surrogate for radioactive species into the crystalline ceramic, to study the courses of the synthesis reactions, and to characterize the final products.
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